1. Field of the Invention
In apparatus working with high pressure, for example containers in extrusion presses where the working pressure can be of the order of magnitude of 1000 - 2000 MPa, leakage past a seal often causes serious secondary damage if the leakage is not discovered and the press stopped in time. That severe damage may arise is easily understood if it is considered that the hydraulic power at the time may be 1 MW and that, in the event of leakage, a considerable part of this power is exerted in the few mm.sup.2 which correspond to the leak. Further, it is difficult to determine from the outside if leakage occurs with the help of the pressure time curve for the main piston as this curve, in case of leakage, normally has the same appearance as when extrusion is taking place.